a) Field of the Invention
This invention relates to a hydrogen absorption alloy for use in hydrogen storage and transportation and an electrode material for a nickel-hydrogen storage battery, and a hydrogen absorption electrode for use as a negative pole of a nickel-hydrogen storage battery.
b) Description of the Related Art
The following four types have been proposed as hydrogen absorption alloys: the AB.sub.5 type, represented by LaNi.sub.5 and MmNi.sub.5 (Mm is a misch metal); the AB.sub.2 type, represented by ZrMn.sub.2 and ZrV.sub.2 ; the AB type, represented by TiFe and TiCo; and the A.sub.2 B type, represented by Mg.sub.2 Ni. They are used in the manufacture of secondary batteries, heat pumps, actuators and fuel tanks for hydrogen automobiles as well as in hydrogen storage and transportation, which use depends on the properties of the alloy, such as the hydrogen absorption temperature and plateau pressure.
In the field of electronic machinery, weight reduction has been in great demand. A battery, which is a part of the machinery, is no exception, and there is also a need for small-sized storage batteries having a higher energy density. As an example, at present, nickel-hydrogen storage batteries, which have a higher energy density than that of a nickel-cadmium batteries, are much sought after in the market place. AB.sub.5 type materials, such as LaNi.sub.5 and MmNi.sub.5, which can absorb and desorb hydrogen electrochemically, or AB.sub.2 type hydrogen absorption alloys having a Laves phase structure, such as ZrMn.sub.2, can be used as the negative pole material of this nickel hydrogen storage battery. At present, MmNi.sub.5 type hydrogen absorption alloys, for which early activation is easy, is put into practical use as a negative pole material for nickel-hydrogen storage batteries. However, in order to obtain higher capacity, further efforts have been made to place a storage-battery utilizing an AB.sub.2 type alloy, in particular, a hydrogen absorption alloy having a Laves phase comprising ZrMn.sub.2 and so on into practical use.
Conventional AB.sub.2 type alloys comprising ZrMn.sub.2 and so on, however, have a problem in that the discharge capacity at high rate discharging will decrease as a result of deterioration due to oxygen in the air. In order to prevent the hydrogen absorption alloy from experiencing atmosphere-induced deterioration, coating- the surface of pulverized alloys with corrosion-resistant nickel or copper has been suggested (Japanese Patent Publications SHO 61-64069, SHO 61-25 101957).
Further, conventional hydrogen absorption alloys have a low hydrogen content when absorbing hydrogen.
For example, the hydrogen content is as low as 1.3 wt. % in Mm--Ni containing AB.sub.5 type alloys and 1.5 wt. %. in Zr--Mn containing AB.sub.2 type alloys.
Furthermore, in manufacturing electrodes, the step of coating metal foil on alloy powder makes the process complicated.